Purines and Pyrimidines-Structures, Examples, and Biological Roles
What are Purines and Pyrimidines?
The purines and pyrimidines are the building blocks of DNA and RNA that form heterocyclic, aromatic compounds as well as belong from two distinct nitrogenous bases. They have many similarities with the chemical anatomy of the organic compound pyridine (C5H5N) and are also closely related to benzene (C6H6) since here: a nitrogen atom replaces one Carbon atom. Purines and Pyrimidines also serve as the basis for several other chemical compounds like caffeine, thiamine, theobromine, etc. The Purines consist of compounds like adenine and guanine, while the pyrimidines comprise of popular compounds like cytosine and thymine.
Pyrimidines primarily have four carbon atoms and two nitrogen atoms, giving it the shape of a ring, as the Nitrogen atoms take the 1st and 3rd place in the ring. The pyrimidines are easily distinguishable like uracil, uric acid, and barbiturates due to such a distinct structure. The pyrimidines are instrumental in the signalling functions of cells, storing energy in the form of phosphates and enzyme regulation, and creating starch and protein.
Purines, on the other hand, consist of pyrimidines and imidazole rings (also known as a five atom ring with two non-consecutive nitrogen atoms). These have a two-ringed composition with nine atoms overall - five-carbon and four nitrogen atoms. Purines are found in a surplus amount in meat, fishes, and grains, and many other food items like starch and proteins. Being a crucial part of the DNA and RNA structure, they have similar functionalities as Pyrimidines.
Structure of Purine and Pyrimidine
(Image to be added soon)
The purines and pyrimidines both contain active molecules like the ones present in drugs and vitamins. The purines have a melting point of 214 °C (487K), and the pyrimidines have a melting point of 20-22°C (room temperature). The purine's molar mass is 120.11 g mol-1, and for pyrimidines, the molar mass is 80.088 g mol-1. They are each synthesized in the lab via the Traube Purine Synthesis method and Biginelli Reaction, respectively.
These compounds contain hydrogen bonding between each other and, therefore, link both the strands present in the DNA double helix structure and make parallel structures between DNA and RNA. In the case of DNA, the purine adenine bond formation takes place with the pyrimidine thymine, while the purine guanine forms a bond with the pyrimidine cytosine. For RNA, the adenine bonds with uracil and guanine need to bond with cytosine. Therefore, to establish DNA or RNA, equal proportions of purines and pyrimidines is a pre-requisite.
Since for DNA and RNA, several other configurations can occur, including that of the methylated pyrimidines, such structures are called 'wobble pairings' as exceptions to the Watson-Crick pairs found in the purine and pyrimidine bases.
Difference Between Purine and Pyrimidine
Purine Catabolism
Purine yields uric acid as the final product in the human body. Simultaneously, other mammals have enzymes like the urate oxidase that form more soluble allantoin as the final product. The guanine nucleotides get hydrolyzed to that of the nucleoside guanosine and are then introduced to phosphorolysis. Since human nucleotidases aren't hyperactive, the AMP is further deaminase to IMP, which is then degraded to yield hypoxanthine.
Conversion of Base to Uric Acid
The adenine and guanine nucleotides have the common intermediate known as xanthine and form xanthine oxidase. When it occurs in the liver, the guanine is deaminated to release ammonia that is carried as glutamine. The xanthine oxidase is present in large amounts in the liver and intestines.
Pyrimidine Catabolism
The Pyrimidines are the final products of the catabolism between the beta-amino acids and the ammonia and carbon dioxide. The pyrimidines that are synthesized from the nucleic acids, with the help of nucleotidases and the pyrimidine nucleoside phosphorylase, form the four-amino group of cytosine and five-methylcytosine. It releases ammonia and carbon dioxide.
Since the purines and pyrimidines are heterocyclic, they can come together to form several nitrogenous bases. However, since purines are made up of two rings instead of one, they have a heavier molecular weight than that of others. The circular ring structure plays its role in the melting points and solubility of these compounds. The aforementioned ways represent how these molecules are synthesized and broken down differently by the body in different places, as the purines are manufactured in the liver and the pyrimidines in the tissues.
FAQs on Purines vs Pyrimidines: Key Differences Explained
1. What is the main structural difference between purines and pyrimidines?
The main difference lies in their chemical structure. Purines, such as Adenine and Guanine, are bicyclic molecules, meaning they have a double-ring structure. In contrast, pyrimidines, like Cytosine, Thymine, and Uracil, are monocyclic, possessing only a single-ring structure. This size difference is crucial for maintaining the uniform width of the DNA double helix.
2. What are the specific examples of purine and pyrimidine bases found in nucleic acids?
The nitrogenous bases in nucleic acids are classified as follows:
- Purines: Adenine (A) and Guanine (G). Both are found in DNA and RNA.
- Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U). Cytosine is present in both DNA and RNA, while Thymine is found only in DNA, and Uracil is found only in RNA.
3. How does the pyrimidine composition differ between a DNA and an RNA molecule?
The key difference in pyrimidine bases between DNA and RNA involves Thymine and Uracil. DNA contains the pyrimidine Thymine (T), which pairs with the purine Adenine. In contrast, RNA contains the pyrimidine Uracil (U) in place of Thymine. Uracil also pairs with Adenine during transcription and in RNA structures.
4. Why must the total number of purines equal the total number of pyrimidines in a DNA double helix?
This principle is known as Chargaff's rule. In a DNA double helix, base pairing is highly specific: a purine on one strand always pairs with a pyrimidine on the opposite strand (Adenine with Thymine, and Guanine with Cytosine). This strict purine-pyrimidine pairing ensures that for every purine, there is a corresponding pyrimidine partner. This 1:1 ratio is essential for maintaining the constant diameter and structural stability of the helix.
5. What is a fundamental difference in the biosynthesis pathway of purines versus pyrimidines?
A major difference lies in how their ring structures are assembled. For pyrimidine synthesis, the single ring is constructed first as an independent molecule (orotate) before being attached to the ribose sugar. Conversely, for purine synthesis, the double-ring structure is not made separately. Instead, it is built step-by-step directly onto the ribose sugar scaffold.
6. Apart from being components of DNA and RNA, what are other vital functions of purines in a cell?
Beyond their role in genetics, purines are essential for cellular metabolism and signalling. They form the core of high-energy molecules like Adenosine Triphosphate (ATP) and Guanosine Triphosphate (GTP), which are the primary energy currency of the cell. Additionally, purine derivatives act as crucial coenzymes (e.g., NAD+, FAD) and signalling molecules (e.g., cyclic AMP).
